Due to the abundance of seawater available and the increasing demand for water suitable for drinking and industrial use, seawater desalination continues to be important. Moreover, large scale, economic, and environmentally sound seawater desalination is especially important, because increases in the population and the continued expansion of various industries have created a growing need for new and inexpensive sources of potable water. Many diverse methods of desalination have been developed including the technologies of distillation, reverse osmosis, freezing, electrodialysis, ion exchange, and forward osmosis. Some of these various methods are detailed in U.S. Pat. No. 3,171,799 to Batchelder; U.S. Pat. No. 3,216,930 to Halff; U.S. Pat. No. 3,670,897 to Frank; and U.S. Pat. No. 5,098,575 to Yaeli.
The primary difficulties presented by these approaches to seawater desalination are adverse environmental impacts and exorbitant water production cost. For example, distillation and reverse osmosis are the most widely employed desalination methods, but both methods produce a process waste stream or brine discharge. Since these processes can only extract a portion (15-50%) of the water from salt water, the remaining seawater, with its increased salinity, is returned to the seawater source. Over a period of time, this waste stream or brine discharge can cause the average salinity of the environment to increase. Moreover, if the desalination process utilizes distillation, the temperature of the waste stream will be higher than the surrounding environs and this may also adversely affect the environment. The long term impact of the waste stream and the brine discharge on the environment is uncertain. But this impact is a significant consideration when constructing seawater desalination plants and has become a primary obstacle to the use of these plants. In addition, these widely employed desalination methods are also expensive. It typically costs twice as much or more to produce fresh water from seawater desalination than when water is obtained by other means. The combination of environmental impact and cost has made sea water desalination prohibitive for all but the most water-scarce environments.
Forward or natural osmosis has also been used for desalination. In general, the forward osmosis desalination process involves a container having two chambers separated by a semi-permeable membrane. One chamber contains sea water. The other chamber contains a concentrated solution that generates a concentration gradient between the saltwater and the concentrated solution. This gradient draws water from the saltwater across the membrane, which selectively permits water to pass, but not salt, into the concentrated solution. Gradually, the water entering the concentrated solution dilutes the solution. The solutes are then removed from the dilute solution to generate potable water.
In particular, U.S. Pat. No. 3,130,156 to Neff and U.S. Pat. No. 3,532,621 to Hough are directed to forward osmosis desalination processes. The Neff patent discloses a forward osmosis process in which a 2 molar solution of ammonium bicarbonate is used to draw water from seawater across a semi-permeable membrane. According to Neff, the dilute solution is then heated to decompose the ammonium bicarbonate solute into its constituent gases. The gases are then released from the solution, leaving behind potable water. The gases released from the solution in the process disclosed in the Neff patent are then compressed or cooled to generate ammonium bicarbonate that is recycled into the concentrated solution in the first step of the process. The Neff patent recognizes that this results in the removal of only a small amount of water from larger quantities of saltwater (low yield). Also, considerable amounts of energy are still needed to vaporize the gas constituents of the solute. Like Neff, the Hough patent discloses a forward osmosis process in which a concentrated solution is used to draw the water from the seawater across a semi-permeable membrane. However, unlike Neff, the Hough patent precipitates the solute out of the solution and recycles the precipitate back into the concentrated solution. According to Hough, expendable or separately recyclable solutes may be needed as reagents for this precipitation and pH adjustment, and further solutes may be needed to balance the pH of the potable water after the precipitation step. These further pH adjustments may result in further precipitation that must be discharged as waste. Thus, these desalination processes fail to remove substantial portions of drinkable water from sea water (high yields) and still require too much energy and result in too high an environmental impact (due to insufficient yield and subsequent brine discharge) to implement them on a larger scale.
In additional applications, it may be desirable to concentrate different species of solute that are present in a solution. In some conventional techniques, heat may be applied to change the phase of a solvent to remove it from the solution, thereby providing the desired species of solute in a higher concentration. Other conventional approaches involve hydraulic pressure driven membrane processes used to push a solvent through a membrane that is permeable to the solvent, but impermeable to the solute of interest. In the context of wastewater treatment, membrane bioreactors have widely replaced traditional secondary wastewater treatment methods for the removal of organic matter from aqueous waste streams. A hydraulic pressure-driven membrane system is typically used to separate water from a biologically active solution in which organic matter is consumed by microorganisms as food, these microorganisms then being separately removed as sludge. The foregoing methods of concentrating solutes have similar drawbacks to those discussed above with respect to desalinization.